It is desirable to be able to heat a metallic workpiece and particularly an elongated metallic workpiece to a greater extent in certain regions than in others or to be able to heat the workpiece in certain regions while other regions are cooled and then ultimately to bring the entire workpiece to a certain temperature. The heating or cooling may cause transformation of the workpiece structure or prevent transformation of the workpiece structure to increase or decrease hardness, to increase or decrease ductility or to change or retain other properties of the workpiece.
There are various known possibilities for heating metallic workpieces, for example, semifinished products, billets to be shaped, pressed articles, metal sections or full sections or tubes to enable parts thereof to participate in thermal modification of the structure or to modify characteristics of the workpiece resulting from a hot-forming process or to otherwise modify mechanical properties. The heating can, for example, be carried out in a continuous furnace whereby the individual workpieces are brought to a uniform shaping temperature which may be independent of the geometric configuration. The continuous furnace usually processes the workpiece for a certain time, which can be considerable depending upon the nature of the workpiece, and can result in the formation of scale on the workpiece which must be removed in a separate step, for example, by sandblasting.
The continuous furnace, of course, must occupy space commensurate with the duration of the treatment and thus the significant size of such a furnace can itself be a drawback.
Metallic workpieces can also be partially or completely heated by inductive processes. With inductive heating, however, especially with long workpieces, temperature gradients can develop which preclude temperature uniformity over different parts of the workpiece.
A metallic workpiece can also be heated conductively, i.e. by the passage of electric current directly through it so that the heat which is generated is a function of the current flow and the resistance of the workpiece. This technique has been used for strip in the form of a coil, bars of metal and like workpieces. In the case of coiled materials, the process can be a continuous one in which the strip is passed over a stretch in which the heating occurs between two electrodes in contact with the strip. Ends of the strip can be spliced together and the strip can be separated for rewinding it in a coil.
In the case of bars and like workpieces, individual workpieces can be heated in succession or a plurality of workpieces can be simultaneously heated. The conductive heating step can be carried out in a small space and at higher rates than furnace heating. However, when the workpiece is to be heated by electric resistance heating and does not have a constant cross section over its length, a problem arises in that at locations of smaller cross section the workpiece heats up much more quickly and much more strongly than at locations of greater cross section. The result is that temperature differences arise in the workpiece and to the point that there may be considerable distortion at the higher temperature smaller regions while at regions of greater cross section, the workpiece may not be brought to a sufficient temperature.
From DE 126 23 20 B a method of heating a steel block is known in which the workpiece is only preheated at its outer regions and the further heating to a thermal deformation temperature is effected by electrical resistance heating to correct nonuniformities of the workpiece structure. In DE 30 26 346 C2, the stretch annealing of workpieces utilizes the supply of electric current through the jaws which engage the workpiece to apply tension thereto so that these jaws also serve as electrodes.
Neither of these references discloses a solution to the above-mentioned problems of conductive heating with workpieces of nonuniform cross section.